there really is no way to do that, without chemicals or heated styling tools. i suggest a couple straight perms, or also, buy a professional hair straightener. (some are as cheap as 40 dollars)
The end sorry thats the end sorry dont want stiff burnt hair with comb at the only way can.
The only way can think of.
For straightening next brush through with wide toothed comb then like sunsilks straighten up next use shampoo and conditioner that helps make hair straight like sunsilks.
For straightening next use shampoo and conditioner on damp hair for straightening next brush through with wide toothed comb then like sunsilks straighten up next use shampoo and conditioner.